Polyester films having excellent transparency, light resistance, water resistance, heat resistance, chemical resistance and mechanical strength are popularly used as films for window application films, which are applied (stuck) on the windows of automobiles, buildings, etc., for the purpose of protection of privacy, impartation of aesthetic effect, adjustment of the sunshine, prevention of scattering of glass pieces in the event of accidental break of glass, etc.
Use of a laminated polyester film having three or more layers containing a dye in the intermediate layer has been proposed as a light-shielding film for windows (Japanese Patent No. 2,699,397 and Japanese Patent Application Laid-Open (KOKAI) No. 8-174750).
In application of these light-shielding films to the windows of automobiles or buildings, their heating insulating effect is found unsatisfactory as they allow passage of light of long wavelength from the sun. Also, the films obtained by depositing a metal on the light-shieldable films are well known, but these films involve the problems that in long time use, the dye or pigment blended for providing the light shielding properties could separate out on the film surface, or transparency is deteriorated by metal deposition.
The present invention has been attained in view of the above circumstances, and its object is to provide a biaxially oriented polyester film suited for window application, which is capable of preventing separation of the dye on the film surface and excels in heat insulation and aesthetic effect in use.